Ethernet Ring Protection Switching, or ERPS, is described, for example, in ITU-T Recommendation G.8032/Y.1344 (08/15), the contents of which are incorporated by reference. G.8032v1 supported a single ring topology and G.8032v2 supports multiple rings/ladder topology. By virtue of its topology, ring based networks, which have learning and flooding on the Layer 2 Ethernet domain, are prone to data packet loops. The G.8032 standard is based on a scheme in which loop-avoidance is the most important criterion as far as protocol specification is concerned. However, during deployment and operation, provisioning related aspects, hardware related aspects, and protocol related aspects all have the potential to cause loops in Ethernet rings. The provisioning related aspects can include, for example, adding removing/modifying ports, Warm/Cold reboot of nodes, changing Virtual Local Area Network (e.g., Broadcast VLAN IDs (B-VID)) or Media Access Control (MAC) (e.g., Destination B-MAC), changing the Admin State of a Ring Protection Group (RPG), changing other provisioning parameters on the RPG (e.g., moving from single card to cross-card and vice-versa, etc.), adding/modifying/removing FlowPoints in active Rings, Virtual Port changes/invalid configuration, incorrect User Provisioning of services (connecting multiple Main Rings at multiple locations), and the like. The hardware related aspects can include issues with blocking Application Programming Interfaces (APIs), issues with forwarding/dropping of Ring-Automatic Protection Switching (R-APS) control frames, issues with flooding/learning behavior, and the like. The protocol related aspects can include, for example, states mismatch/getting stuck due to R-APS messages not getting forwarded (due to error or mis-provisioning in upstream/downstream nodes), blocking/forwarding behavior change due to timing related issues, event handling (Missing Port/Continuity Check Message (CCM) Up/Down detection), events detected on the edge of timers (hold-off, guard etc.).
There are conventional mitigation strategies to address some of the aforementioned causes of loops. The G.8032 protocol, by definition, is designed around loop-avoidance, and conformance testing the protocol stack would ensure the protocol related issues are covered to a large extent. For scenarios not specified well by the standard, a conservative approach can include ports coming up always blocked, usage of guard timers to ensure frames that cross each other are dropped, dropping R-APS control frames checking for self Node ID in the R-APS packet, etc. to name a few. Comprehensive System testing and automation testing can catch some of the errors related to user provisioning, and standalone hardware testing can ensure the Hardware/Switch related API's work as specified.
In spite of the conventional mitigation strategies, there can be instances where loops occur, and the adverse condition typically manifests as one of the below:    1. All ports in the ring show as unblocked (forwarding) on in a Network Management System (NMS), i.e., no blocked port on any of the participating nodes in the ring;    2. One (or more) port(s) show blocked status in the NMS, but due to internal (hardware/software) issues, the ring is not really blocked. (In essence, providing misleading information); and    3. Invalid User Provisioning: for example, connecting two Main Rings at multiple Nodes (creating loops depending on Blocking positions of Both Rings.).
These adverse conditions can bring down not only the rings involved but sometimes the line card/system and the network as well.
Another conventional approach is MAC movement which monitors a L2 MAC table to detect recurring movement of MACs between different ports. MAC movement is normal, but recurring movement is not. Detection of a loop using MAC movement requires a centralized MAC learning analysis which may not be possible in distributed systems. Also, some loops may occur that do not exhibit this condition and cannot be detected by MAC movement. With fast ERP solutions, there is a need for loop detection as a second and more robust line of defense.